So far, a method for supplying reaction gases in a catalytic gas-phase oxidation process in which a material to be oxidized and a gas containing molecular oxygen are mixed and the resultant mixture is supplied to a catalytic gas-phase oxidation reactor and subjected to a catalytic gas-phase oxidation reaction has been known. For example, a method of producing methacrolein by reacting isobutylene and oxygen and a method of producing methacrylic acid by reacting methacrolein and oxygen are exemplified (cf. Japanese Patent Application, First Publication No. 2000-356450).
In the case of mixing a material to be oxidized and a gas containing molecular oxygen and performing a catalytic gas-phase oxidation reaction, there is mostly a peculiar explosion range in a compositional range resulting from a combination of a material to be oxidized and molecular oxygen. In the present invention, the explosion range means a compositional range in which the material to be oxidized and oxygen possibly react to cause explosion. Besides an explosion range can be termed as a combustion range.
On the other hand, in the case of a catalytic gas-phase oxidation reaction which produces methacrolein or methacrylic acid by using isobutylene or methacrolein as a material to be oxidized, it is important to avoid giving a catalyst a reducing atmosphere from the view point of the catalyst life, so that the composition of a gas at the inlet of a catalytic gas-phase oxidation reactor tends to become near an explosion range, even though it is outside the explosion range.
Further, in the prior art, an unchanged fixed point in a concentration composition was set up taking a safety into consideration, however, when changing as the occasion demands, especially a concentration composition while continuing an operation, there was no indication about a method of securing safety as for a composition in the course of the change.